Referring to FIG. 1, there is shown a schematic cross sectional view of a reaction chamber 40 of a vertical type low pressure CVD apparatus, into which a standard boat 5 mounting therein a plurality of stacked Si wafers 8 is loaded, for use in forming Si or SiGe epitaxial growth films on the Si wafers 8. Further, there are shown in FIGS. 2A and 2B, a top cross sectional view of the standard boat 5 in FIG. 1 and a partial front cross sectional view thereof, respectively.
As shown in FIG. 1, the reaction chamber 40 includes a base portion 1 at its lower part, a gas supply port 2 installed at the base portion 1, and an outer tube 3 and an inner tube 4 at the upper part of the base portion 1, the inner tube 4 being located at inside the outer tube 3. The base portion 1 is further provided with an exhaust port 7 in such a manner that the exhaust port 7 communicates with the space provided between the outer tube 3 and the inner tube 4. The standard boat 5 is loaded in the inner tube 4.
The standard boat 5 has, e.g., four vertically arranged support rods 6 as shown in FIG. 2A. The support rods 6 have a plurality of wafer mount grooves 9 for horizontally holding a number of Si wafers 8 with an identical gap therebetween as shown in FIG. 2B.
In such a vertical type low pressure CVD apparatus, the Si or the SiGe epitaxial growth films are formed on the Si wafers 8 by supplying and exhausting the film forming gas, e.g., SiH4 or SiH4 and GeH4, to and from the reaction chamber 40 via the gas supply port 2 and the exhaust port 7.
Boron(B) can be doped into the Si or the SiGe epitaxial growth films, by introducing a doping gas B2H6 into the reaction chamber 40 through the gas supply port 2, together with the film forming gas of SiH4 or SiH4 and GeH4.
Since, however, the gas phase reaction of B2H6 is rather strong, B2H6 reacts not only with Si wafers 8 but also in other portions of the reaction chamber 40. Further, B is readily doped into the Si or the SiGe epitaxial growth films. This results in a great consumption rate of B2H6. Therefore, the respective Si wafers 8 located closer to the downstream side of the gas flow in the reaction chamber 40 tend to have lower concentrations of B in the Si or the SiGe epitaxial growth films formed thereon, resulting in the non-uniformity in the boron concentrations in the Si or the SiGe epitaxial growth films depending on the location of the Si wafers 8 in the reaction chamber 40. For this reason, it may become necessary to install in the reaction chamber 40 a number of gas supplement nozzles (not shown) for compensating for the rapid depletion of B2H6 gas. However, the increased number of gas supplement nozzles renders the vertical type low pressure CVD apparatus rather costly.
Further, when gaps between the Si wafers 8 mounted in the standard boat 5 are small, it is difficult for sufficient amounts of B2H6 to diffuse to the central portions of the respective Si wafers 8. This causes non-uniformity in the concentration of B even within a same Si wafer, exhibiting a concentration gradient of B in the Si or the SiGe epitaxial growth film lowering from the peripheral portion towards the central portion thereof.
For instance, when SiGe epitaxial growth films were grown on the Si wafers 8 by loading into the reaction chamber 40 the Si wafers 8 of a diameter of 200 mm mounted in the standard boat 5 with a gap of 5 mm therebetween and then introducing thereinto B2H6 as the doping gas, the sheet resistances, at the location of A to I shown in FIG. 3, of a SiGe epitaxial growth film formed on a Si wafer 8 were 1498 Ω/□, 2640 Ω/□, 2800 Ω/□, 2510 Ω/□, 1463 Ω/□, 1633 Ω/□, 2600 Ω/□, 2650 Ω/□, 2070 Ω/□ respectively, showing the variation of sheet resistances of ±30.3%. In general, when B is doped into the SiGe epitaxial growth film, the sheet resistances thereof become greater as the concentration of B in the SiGe epitaxial growth film becomes lower. Therefore, from the data described above, it is evident that the concentration of B is not uniform at the surface of a SiGe epitaxial film.
In order to solve the foregoing problems, it may be necessary to increase the gaps between the Si wafers 8 mounted in the standard boat 5. For example, when SiGe epitaxial growth films were grown on the Si wafers 8 by loading into the reaction chamber 40 the Si wafers 8 having a diameter of 200 mm mounted in the standard boat 5 with a gap of 10.5 mm therebetween and then introducing thereinto B2H6 as the doping gas, the sheet resistances, at the locations of A to I shown in FIG. 3, of the SiGe epitaxial growth film formed on a Si wafer 8 were 932 Ω/□, 1299 Ω/□, 1348 Ω/□, 1272 Ω/□, 879 Ω/□, 985 Ω/□, 1295 Ω/□, 1324 Ω/□, 1176 Ω/□ with the variation of the sheet resistances of ±20.1%. From this, it can be seen that, by increasing the gap between the Si wafers 8 loaded into the reaction chamber 40, the variation of the sheet resistances of the SiGe epitaxial growth films formed thereon decreases, and the uniformity of the concentration of B in the SiGe epitaxial growth films is improved.
However, when the gap between the Si wafers increases, the number of Si wafers on which the process of the film formation can be carried out at one time is reduced. For instance, when the gap between the Si wafers mounted in the standard boat become double, the number of Si wafers on which the process of the film formation can be carried out at one time is reduced to a half.
In such a standard boat 5, since B2H6 is also consumed at the support rods 6 thereof, the concentration of B becomes low in the Si or the SiGe epitaxial growth films at the portions of the Si wafers 8 close to the support rods 6, aggravating the uniformity of concentration of B in the growth films. In order to solve the afore mentioned problems, a ring boat having rings for separating the Si wafers 8 away from the support rods can be employed.
Referring to FIGS. 4A and 4B, there are shown a top cross sectional view and a partial front cross sectional view of a ring boat. As shown, the ring boat includes, e.g., four (see FIG. 4A) vertically arranged support rods 11. The support rods 11 are provided with a plurality of rings 12, each of rings 12 being provided with a plurality of, e.g., three, pins 13 for mounting one Si wafer 8 thereon.
In such a ring boat, since the distance between the support rods 11 and the periphery of Si wafers 8 is increased, it is possible to reduce the adverse effect of the support rods 11 against the uniformity of concentration of B in the Si or the SiGe epitaxial growth films formed on the Si wafers 8. For instance, when B doped SiGe epitaxial growth films were grown on the Si wafers 8 by loading into the reaction chamber 40 the Si wafers 8 having a diameter of 200 mm mounted with a gap of 11.5 mm therebetween and then introducing thereinto B2H6 as the doping gas, the sheet resistances, at the locations of A to I shown in FIG. 3, of the SiGe epitaxial growth film formed a Si wafer 8 were 266 Ω/□, 278 Ω/□, 287 Ω/□, 262 Ω/□, 236 Ω/□, 251 Ω/□, 266 Ω/□, 286 Ω/□, 308 Ω/□ with the variation of the sheet resistances of ±9.4%.
However, the price of the ring boats described above is considerably higher than that of the standard boats. Further, since the ring boat has a greater reaction area than the standard boat, the consumption rate of B2H6 increases. As a consequence, the adverse effect on the uniformity of B concentration depending on the relative locations of wafers with respect to the gas flow direction becomes worse in the case of ring boat, compared with that of standard boat, necessitating the installation of even a larger number of gas supplement nozzles in the CVD apparatus.